The present invention relates to a prosthetic acetabular cup impaction plate for use with a particular type of acetabular cup.
Prosthetic acetabular cup inserter and impactors are used to implant prosthetic acetabular cups into the cavity of a patient's hip, and generally comprise an elongate body with a cup engaging head at a first end thereof, and a handle and impaction anvil at a second end thereof. The surgeon releasably fits a cup implant to the cup engaging head, and then positions the cup inside the patient's hip. He then applies a hammering force to the impaction anvil to secure the cup in place, before releasing the cup from the cup engaging head.
In some cases the impaction force is imparted to the cup via an impaction plate provided at the cup engaging head. Such plates can comprise an annular cup engaging wall adapted to engage a rim of the cup such that the impaction force can be imparted thereto. Traditional acetabular cups are regular in shape, and comprise a rim in a single plane. An impaction plate suitable for use with such a cup construction would therefore comprise a cup engaging wall with an upper surface which is also in a single plane.
However, the peripheral rim of the natural acetabulum has a contoured shape and does not extend through a single plane. Considered perpendicular to the proximal/distal axis of the hip joint, the shape of the acetabular rim comprises three convex portions which extend between the ilium bone and the pubis bone, between the pubis bone and the ischium bone and between the ischium bone and the ilium bone. Of these three convex portions, that between the pubis and the ischium bones extends proximally to a greater extent than the other two. Further, the convex portion between the ilium and ischium bones is of a greater length than the other two. In addition, the intervening concave sections of the rim adjacent to the ischium and ilium bones extend distally to a greater extent than the remainder of the rim. As a result of this contoured shape, the angle of movement of the natural hip joint at certain positions of the femur is greater than that provided by a traditional prosthetic acetabular cup with a rim in a single plane. A further difficulty with such cups is that if they are not correctly positioned in the acetabulum, part of the rim can extend beyond the acetabulum. This can cause pain due to the rim conflicting with the psoas muscle which extends from the acetabulum to the femur, or with other muscles or tissues.
In order to solve some of the above problems U.S. Pat. No. 7,833,276 in the name of the applicant discloses a prosthetic acetabular cup with a rim shaped to mimic the contours of the natural shape of an acetabulum. The rim comprises spaced convex ilium, ischium and pubis portions, with concave portions in between. U.S. Pat. No. 7,833,276 also refers to an impaction plate comprising an annular cup engaging wall with an upper surface shaped to complement that of the rim of the cup. Such an impaction plate is shown more clearly in U.S. Patent Application Publication Nos. 20120136361, 20120136360 and 20110130763 in the name of the applicant. These applications relate to prosthetic acetabular cup inserter and impactors which are for use with acetabular cups such as shown in U.S. Pat. No. 7,833,276, and therefore may comprise impaction plates with annular cup engaging walls with a shape which complements that of the rim of the cup. The disclosure of U.S. Pat. No. 7,833,276 is incorporated herein by reference.
A problem with impaction plates with cup engaging surfaces shaped to complement the rims of acetabular cups designed to mimic the acetabulum, is that they are side specific. The natural shapes of the right and left acetabular rims mirror one another, but as the shapes are not symmetrical a cup designed to mimic the right acetabulum will not mimic the left. As a result, specific right and left hand side cups are provided, which necessitate specific right and left hand side impaction plates. The manufacture of side specific cups is not a particular problem, as a cup is required for each procedure in any case. However, the requirement for side specific impaction plates is a problem, because it adds a significant complication to the inserter and impactor tooling. Where in the past a single tool for repeated use in right and left side procedures could be provided, the side specific cups require either two separate side specific inserter and impactor tools, or a tool with interchangeable side specific impaction plates.
The present invention is intended to overcome some of the above problems.